The present invention relates to the biological removal of carbonaceous matter and other pollutants from wastewater. More specifically, the invention provides a method for the treatment of wastewater utilizing partially submerged rotatable biological contactors to effect the removal of an optimum amount of carbonaceous matter per unit area of surface provided in the form of biological contactors. Further, the invention provides operating parameters for minimizing the size of a wastewater treatment plant employing rotating biological contactors for the removal of a desired amount of carbonaceous matter.
The concentration of carbonaceous pollutants in wastewater in usually expressed in terms of BOD.sub.5 (biochemical oxygen demand) which is a term commonly known and used in wastewater treatment. "BOD.sub.5 " is the amount of molecular oxygen consumed by a fluid under predetermined established conditions at 68.degree. F. during a 5 day period expressed in terms of (mg/liter). "Filtrate BOD.sub.5 " is used to express the amount of soluble and colloidal carbonaceous matter present in the wastewater after filtration of all particles larger than about 0.5 micron. The expression, "filtrate BOD.sub.5 loading rate," is used herein to express the amount of such filtrate BOD.sub.5 supplied per day per 1,000 square feet of contactor surface i.e. (lbs/day/1,000 sq. ft).
The use of rotatable biological contactors that are partially submerged in the wastewater and rotated at a predetermined rate to alternately expose the surface upon which biological slimes grow to the atmosphere and immerse them in the wastewater are well known. Representative U.S. patents disclosing treatment systems employing rotating contactors to grow biological slimes on the contactor surface are Torpey U.S. Pat. No. 3,575,849; El Naggar U.S. Pat. No. 3,335,081; Simpson U.S. Pat. No. 3,466,241; and Hartman et al U.S. Pat. No. 3,389,798 among others. Although rotating contactor treatment systems are very effective and efficient in removing carbonaceous pollutants from wastewater in cold weather, i.e., a wastewater temperature of less than about 60.degree. F., their efficiency may be substantially reduced during warm weather, i.e., a wastewater temperature of more than about 60.degree. F. since deleterious organisms such as beggiatoa and other filamenteous organisms that do not consume carbonaceous material, proliferate and successfully displace carbonaceous consuming organisms from the contactor surface area. Consequently, a substantial portion of the contactor surface area is lost to the oxidation of reduced sulphur compounds rather than the removal of carbonaceous pollutants.
It is a primary objective of the invention to provide a wastewater treatment system of the foregoing type wherein such growth of deleterious organisms on the contactor surface is eliminated during warm weather operation. It is further an objective of the invention to define operating parameters for a treatment system utilizing rotating contactors wherein the contactor surface area required for a desired BOD.sub.5 removal is minimized.